5.4 Pressure

Question 1

Pressure

Answer 1

The concentration of a force or the force per unit area.
P = F/A

P = pressure (pa)
F = Force (N)
A = area (m^2)
If a force is spread over a large area it will result in a small pressure
If it is spread over a small area it will result in a large pressure

Question 2

Atmospheric pressure

Answer 2

The atmosphere extends more than 100 km into space and becomes less dense with increasing altitude (height above sea level).
This means that the pressure becomes less too.
This is due to air molecules colliding with a surface creating atmospheric pressure.
These molecules create a force per area of the surface which creates the pressure.
The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases.
Therefore, as height increases, there is always less air above a surface than there is at a lower height and the atmospheric pressure decreases with an increase in height

Question 3

Liquid pressure

Answer 3

In a liquid, the pressure at a point increases with the height of the column of liquid about that point.
If there is more liquid above that point, then the pressure is more.
This is because the pressure in a liquid is caused by the weight of the liquid pushing against objects immersed in the liquid.
As the liquid becomes deeper, the amount of liquid (and hence the weight) increases which causes the pressure to increase.
p = ρhg

p = pressure in pascals (Pa)
h = height of the column in metres (m)
ρ = density of the liquid in kilograms per metre cubed (kg/m3)
g = gravitational field strength on Earth in newtons per kilogram (N/kg)

Question 4

Upthrust

Answer 4

A partially submerged object (an object that’s not fully in a liquid) will experience greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. We call this force upthrust.
The upthrust that acts on an object is equal to the weight of the liquid that has been forced away (displaced) by that object.
If the object’s weight is equal to the upthrust, then the forces balance and the object will float in the liquid.
If the object’s weight is greater than the upthrust, then the object will sink.

Question 5

Depth control of submarine

Answer 5

Submarines use the balance between upthrust and weight to control their depth in water.
When a submarine wants to sink, it fills its tanks with water to increase its weight.
This means the submarine’s weight is greater than the upthrust and it sinks.
When a submarine wants to come up to the surface, it fills its tanks with compressed air to reduce its weight.
Weight becomes less than upthrust so the submarine rises.